My furnace makes a vibrating noise when the vent motor starts and it continues until the blower motor starts to run. It may still be there after the blower starts but the sound of the blower covers it. The noise can be heard from the registers but is not very noticable if you stand in front of the furnace.

I've removed the furnace cover and observed the vent motor and it seems to turn freely. After the power shuts down to the motor it continues to wind down at a normal rate so I'm inclined to think it may be something in the blower assembly??

First, is the vent fan a squirell cage type? If so there may be a balancing weight that has fallen off it. (They are just little metal clips slipped on a fin here and there.)
Another cause is worn bushings in the vent fan motor. See if the motor shaft can be moved up and down with the motor off. The remedy for this is a new motor.
Glenn

I removed the unit and removed the screws on the back side of the blower housing however it will not separate. I believe the halves may be glued together. The motor has in/out play on the shaft but no noticeable lateral play. I believe the blower must be replaced as a complete unit. I found what looks like the unit at www.americanhvacparts.com and it is called "Inducer blower motor" and the motor and blower are together as a single unit.

Since the cost of this unit is $267 I am wondering if anyone has had the same problem and if the unit needs to be replaced soon.

Hey Mike:
Sorry my ideas didn't work; but hey, I could listen to a lot of vibration for $267. As long as it is working, let 'er ride.
Now, you need to open the top panel of the furnace and you'll find the blower and motor that circulates the air. I still say it sounds like worn bushings or it could be a piece of loose insulation rubbing against the fan.
Glenn